Be-si alloy for structural parts

ABSTRACT

Parts formed from a beryllium-silicon alloy containing approximately 62 weight percent silicon are readily castable without the formation of large porosities or shrinkage cavities on cooling. They are characterized by a high ratio of elastic modulus to density and a high ratio of thermal conductivity to thermal expansion, are relatively homogeneous and isotropic throughout, and are surprisingly less brittle than either of their component metals. They are especially useful as mirror substrates.

United States Patent Kaufmann [451 Mar. 28, 1972 [54] BE-SI ALLOY FOR STRUCTURAL PARTS [72] Inventor:

[73] Assignee:

Albert R. Kaufmann, Lexington, Mass.

Whittaker Corporation, Nuclear Metals Div., West Concord, Mass.

221 Filed: 0ct.29, 1968 21 Appl.No.: 771,425

Max Hansen, Constitution of Binary Alloys, N.Y.: Mc- Graw-l-Iill 2nd ed., I958, pages 296- 297 relied on.

Primary ExaminerL. Dewayne Rutledge Assistant Examiner-E. L. Weise Attorney-Donald E. Nist and Jay I-I. Quartz [57] ABSTRACT Parts formed from a beryllium-silicon alloy containing approximately 62 weight percent silicon are readily castable without the formation of large porosities or shrinkage cavities on cooling. They are characterized by a high ratio of elastic modulus to density and a high ratio of thermal conductivity to thermal expansion, are relatively homogeneous and isotropic throughout, and are surprisingly less brittle than either of their component metals. They are especially useful as mirror substrates.

22 Claims, 3 Drawing Figures 'P'ATEMTED MAP. 2a 1972 ATOMIC PER CENT SILICON FIG.I

FIG.2

N m mM W m R T R E B L A 3 G F M a: w

ATTORNEYS BE-SI ALLOY FOR STRUCTURAL PARTS BACKGROUND OF THE INVENTION A. Field of the Invention The invention relates to castable parts having a high elastic modulus-to-density ratio. More particularly, it relates to parts of high modulus-to-density ratio formed from a beryllium-silicon alloy. These parts are especially useful as mirror substrates.

B. Prior Art Parts having a high elastic modulus-to-density ratio are generally desirable for most applications since a smaller amount of material can be used in forming a part of given stiffness characteristics, thereby providing a direct reduction in its cost. Further, in some applications, for example in airborne or aerospace applications, the availability of a lightweight part of high elastic modulus to density ratio is often of critical importance. (The word "part herein designates an object having a shape adapted to accomplish a particular function and is intended to exclude shapes arising solely in forming the alloy or used solely for tensile-testing purposes.)

Parts which are castable, as opposed to those which must be machined or otherwise worked into shape, are especially desirable since casting techniques are sufficiently advanced to provide important economies in the manufacturing process as compared with machining or other working methods. In addition to the direct savings obtained from eliminating or reducing the amount of machining or working required, parts that can be cast nearly to shape obviate the removal of extensive quantities of material which would otherwise appear as scrap in the finishing operations. When materials of relatively high cost per pound are used, these savings can be substantial.

Not all materials are readily castable. Most materials shrink on freezing; in some materials such as beryllium this shrinkage generates large shrinkage cavities (pipe") within the casting which hasten the failure of the cast part when it is subjected to large loads. Such shrinkage cavities within the part can be avoided only by complicated mold designs which require pouring much more metal than is needed for the final object. Even these precautions may not be successful in thin-walled regions of the casting. Materials of this sort are generally not suitable for forming parts directly by casting, and must be extensively worked after casting to reduce the cavity size and thereby impart additional strength to the material over that obtainable in the as-cast condition. This greatly increases the cost of the finished parts.

In many applications it is desirable to form a structure from a material having a high ratio of thermal conductivity to coefficient of thermal expansion (IO/a. Additionally, it is often desired that such a material also possess a high elastic modulus-to-density ratio (E)/p. An example of a material fulfilling these requirements is beryllium for which these factors are approximately 9.5 X b.t.u./ft./hr. and 6.5 X 10 in. respectively.

An important example of an application requiring high E/p and high K/a is in the field of mirrors. Frequently, the mirror area is quite extensive and the substrate on which the mirror surface is formed is required to withstand rather severe bending loads in use. Further, due to the extensive surface area of the mirror, it is necessary that the supporting structure not become unduly distorted due to temperature changes or due to uneven temperature distributions over the mirror surface.

Because of its high E/p and high K/a, beryllium would nor mally be excellent for the structural member of the mirror. However, despite its high elastic modulus, pure beryllium is quite brittle and is very susceptible to brittle fracture due to the presence of large grains in cast beryllium. Further, pure beryllium is not readily castable, since it forms large shrinkage cavities on cooling. These cavities could further contribute to the early failure of the cast material when it is placed under stress.

Because of the difficulty of casting parts of beryllium and their extreme brittleness, it is commercial practice to prepare beryllium by powder metallurgy. This involves hot pressing of large blocks of the metal, from which a desired final object can be obtained only be extensive machining. The cost of a hot pressed beryllium powder block is about $80. per pound and the cost of finished machined parts can be as high as $300. to $400. per pound. Furthermore, the salvage value of beryllium machining chips is only $5. to $10. per pound. These factors severely limit the use of beryllium in the formation of most parts which could otherwise benefit from its properties.

A further and often unacceptable characteristic of the beryllium crystal is that it is highly anisotropic, that is, its properties vary with direction within the material. Even the powder metallurgy form of beryllium exhibits significant anisotropy because the individual crystals within the body are not randomly oriented. In any form of wrought beryllium there is a marked preferred orientation of the crystals and consequently severe anisotropy. This factor alone is often enough to prohibit the use of beryllium in mirrors since a slight anisotropy in the structure on which the mirror surface is formed can lead to distortions in the mirror shape and thus in the image reflected by the mirror. These distortions, though small, may be of a magnitude comparable to the wavelength of light so as to generate undesired interference patterns.

Previous attempts have been made to alloy beryllium with other materials to minimize one or more of its faults and to enhance its useful characteristics. However, in the materials heretofore utilized in combination with the beryllium, some of the materials, such as copper, greatly increase the density of the alloy so that its index E/p is greatly decreased; they may also greatly increase its coefficient of thermal expansion so as to destroy its usefulness for such critical applications as mirrors. Additionally, most of the materials heretofore used as alloying materials form brittle intermetallic compounds with the beryllium; these compounds greatly weaken the alloy and the structure formed from it.

BRIEF SUMMARY OF THE INVENTION A. Objects of the Invention Accordingly, it is an object of the invention to provide a part of high elastic modulus-to-density ratio that is readily castable.

Further, it is an object of the invention to provide a castable part of relatively high ratio of thermal conductivity to thermal expansion.

A further object of the invention is to provide a castable part having a high ratio of elastic modulus to density and an increased resistance to brittle fracture.

Yet another object of the invention is to provide a castable, relatively impact-resistant part of high ratio of modulus to density that is readily castable without the formation of large porosities.

Yet another object of the invention is to provide a castable, impact-resistant part having a high ratio of modulus to density that is relatively homogeneous and isotropic throughout.

Yet another object of the invention is to provide a castable part of a beryllium alloy that is resistant to brittle failure.

Still a further object of the invention is to provide a castable part from an isotropic, homogeneous, microporous, beryllium alloy.

Still another object of the invention is to provide a castable mirror substrate having a high elastic modulus-to-density ratio and relatively good resistance to brittle fracture.

Still a further object of the invention is to provide a castable mirror substrate that is isotropic throughout and that is characterized by the absence of large shrinkage cavities in the substrate as cast.

Yet another object of the invention is to provide a castable mirror substrate formed from a beryllium alloy utilizing relatively low cost materials.

Yet another object of the invention is to provide a castable mirror substrate formed from an isotropic, homogeneous, microporous, beryllium alloy. A further object of the invention is to provide a method of forming a uniformly dense part with a resistance to brittle impact comparable to that of beryllium from a silicon-beryllium alloy.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises an article of manufacture possessing the features, properties, and the relation of elements which will be exemplified in the article hereinafter described, and the scope of the invention will be indicated in the claims.

B. Brief Description of the Invention l have found that a beryllium-silicon alloy having a composition in the neighborhood of the eutectic composition ranging from about 30% to 70% by weight of silicon (preferably from about 50% to 62% by weight of silicon) has unexpectedly high resistance toimpact as compared with the brittleness of either of its components.

This finding is in marked contrast to the conclusions of those who have previously prepared beryllium-silicon alloys. Heretofore it has been assumed that the alloying of silicon and beryllium, which are both highly brittle, would result in an alloy at least as brittle as either of its starting materials. See, for example, the article on the beryllium-silicon system by G. Massing and O. Dahl entitled Silizium-Berylliumlegierungen and appearing inWiss. Veroffentl. Siemens Konzern, \ol. 8 (1929) p. 225-256, which discusses beryllium-silicon alloys in general and which states that they are extraordinarily brittle," this conclusion being considered completely forseeable in view of the brittleness of the starting materials. Accordingly, although beryllium-silicon alloys have heretofore been prepared in the laboratory for investigatory purposes, they have not been applied to the formation of structural parts.

In addition, I have found that such an alloy possesses a desirably high ratio of elastic modulus to density, has a large ratio of thermal conductivity to thermal expansion, and, within a preferred compositional range, is relatively homogeneous and isotropic throughout. This is in strong contrast to the anisotropy which characterizes its beryllium and silicon starting materials. Of equal importance, I have found that such an alloy is readily cast into a large number of shapes without the formation of large shrinkage cavities on cooling of the melt.

This alloy appears ideally suited to the economic construction of mirrors and other shapes and structures which require these very properties. Further, in addition to its advantageous properties, important economies are achievable due to the castability of the alloy and to the ready availability of silicon at low cost. For example, silicon can readily be obtained for from l cents to 20 cents per pound, while commercially pure beryllium pebbles cost on the order of from $50-$60. per pound. Thus, the replacement of beryllium with silicon in the alloy leads to dramatic cost savings. These advantages make the alloy a commercially significant material with potentially numerous applications.

SPECIFIC DESCRIPTION OF THE INVENTION Other objects, features and advantages of the invention will more readily be understood and appreciated in the following detailed description of a preferred embodiment therefor selected for purposes of illustration and shown in the accompanying drawing in which:

FIG. 1 is a diagrammatic view in perspective of amirror which may advantageously be formed in accordance with the present invention;

FIG. 2 is a diagram of a binary phase diagram for the beryllium-silicon alloy system; and

FIG. 3 is a photomicrograph of the beryllium-silicon alloy formed close to the eutectic composition.

The beryllium-silicon alloy described herein may be used to form any of a number of structures but is especially useful in forming structures which require a unique combination of characteristics not heretofore believed obtainable. Such a structure is shown in FIG. 1 which depicts a mirror 10 having a base or substrate 12 on which a reflecting surface 14 is formed by polishing the substrate, by vapor-depositing a reflecting surface on the substrate, or by other well-known means. Struts 16 provide structural support to the mirror and also serve as connecting elements for fixing the mirror to other objects.

In order to obtain a mirror of uniform properties which do not vary substantially with temperature, time, or direction, the substrate 12, which may have a substantial surface area, must meet many stringent requirements. First, it must have a relatively low coefficient of thermal expansion in relation to the amount of heat conducted by it. In other words, the ratio K/a must be relatively high. This insures that the material will be able to withstand a wide range of temperatures without undergoing a substantial deformation which could lead to severe optical distortions.

Second, the material of which the substrate 12 is composed should be substantially isotropic, that is, its properties should be substantially independent of direction within the substrate. This insures that the changes in dimension of the substrate with respect to temperature, loading conditions, etc., will be substantially uniform with respect to direction so that optical symmetry will be preserved.

Third, the substrate 12 should be formed of a material that is lightweight yet of relatively high stiffness in relation to its weight. Specifically, the material should have a relatively high E/p. This allows one to use a lighter-weight structure to achieve the same strength characteristics and in many cases allows the use of larger and thinner sections than heretofore previously obtainable.

Fourth, the substrate 12 should be simple and inexpensive to manufacture. This implies that the materials from which the substrate are formed should themselves be inexpensive and also implies that the manufacturing processes required to form the structure into the desired shape be simple and inexpensive. Preferably, the substrate should be formed from a material which is capable of being cast into its final shape with little further working required.

I have found that a beryllium-silicon alloy within a certain preferred range of composition possesses the unique combination of properties called for above. FIG. 2 is a phase diagram of the beryllium-silicon system showing the variation of the phases of the alloy with temperature and with composition. The circles represent the data points from which the phase diagram was obtained. As may be seen from this diagram, the eutectic composition occurs at a point corresponding to ap proximately 62% by weight of silicon and 38% by weight of beryllium; the eutectic temperature is approximately l,090 C. This diagram is given in Massing and Dahl, supra.

FIG. 3 is a photomicrograph, taken at a magnification of X, of beryllium-silicon alloy near the eutectic having a composition of approximately 6l w/o (wt. silicon and 39 w/o beryllium. The alloy was formed in a beryllia-lined mold. The specimen was etched and polished in the usual manner before the photomicrograph was taken. The light areas are beryllium, while the grey areas are silicon. It will readily be seen that the alloy has a predominantly fine microstructure, with the silicon and beryllium intimately mixed with each other. The presence of small dendrites of primary beryllium indicates that the alloy composition is slightly on the beryllium side of the eutectic.

A few irregular porosities (which appear as black marks in FIG. 3) may be observed in the alloy; these are concentrated primarily along grain boundaries. These porosities are caused by shrinkage of the melt on cooling. They are relatively small in size, especially when compared with the equivalent shrinkage cavities which occur on the cooling either of pure beryllium or of pure silicon.

The alloy depicted in FIG. 3 has a density of 0.0762 lb./in., a Young's modulus E of 26810 lb./in., an ultimate tensile strength of 17,500 lb./in. a microyield strength of 3,800 lb./in. for a residual strain of 10' in., and a coefficient of thermal expansion of 36 F. The material thus has an E/p of 36210 in. and a K/a of 12.7.10 B.t.u./ft./hr. The comparable figures for commercially pure beryllium are 657- in. and 944-10 B.t.u./ft./I-lr. respectively for Brush 8-200 beryllium. Thus, the alloy has an elastic modulus-to-density ratio that compares favorably with that of beryllium and a ratio of thermal conductivity to thermal expansion that is greater than that of beryllium. In addition, it is significantly less brittle than cast beryllium.

Careful examination of photomicrographs such as the one shown in FIG. 3 has lead to the conclusion that the berylliumsilicon alloy has a structure near the eutectic composition such that the silicon and beryllium each exist in the form of fine, long, continuously-connected strands of material, each forming a spongelike network enmeshed within, and surrounding, the spongelike network of the complementary material. Several of the remarkable properties of the alloy are believed to follow from this structure. In particular, the fine interweaving of the beryllium and silicon networks creates a microporous structure which causes a more uniform distribution of porosities or shrinkage cavities over a much larger volume, thereby preventing the concentration of large shrinkage cavities at isolated points throughout the volume. Further, the intimate mixture of the two components is believed to explain the surprising resistance of the silicon-beryllium alloy to brittle fracture. Because of the random character of the structure which causes the suppression of large, coarse grains of beryllium or silicon in its formation, the alloy is more readily able to withstand crack propagation on impact than would be the case with a coarser-grained alloy.

Further, due to the random nature of the cellular structure of the alloy, there are no preferred" directions within the alloy structure. Accordingly, the material is surprisingly isotropic throughout; this is in strong contrast to the anisotropy that prevails in pure beryllium.

In addition to the increased strength which the preclusion of large shrinkage cavities leads to, the microporosity of the alloy allows the ready formation of a wide variety of parts ranging from the simple to the complex, by a simple casting process. Thus, expensive machining and forming operations can either be eliminated entirely or substantially reduced in extent. Further, the removal of large amounts of material which would necessarily occur in forming many parts from a cast ingot is either obviated in its entirety or at least largely eliminated by casting the alloy to its desired shape as closely as possible.

The properties of the beryllium-silicon alloy vary, as may be expected, with the percentage of silicon in the alloy, but show increasingly undesirable behavior (from the viewpoint of high strength) as the amount of silicon is increased to any substantial extent above the eutectic composition. For example, in a composition of approximately 67 weight percent silicon and 33 weight percent beryllium, long sheets ofprimary silicon appear along which the alloy fractures easily. Accordingly, it will generally be preferable to maintain the percentage of silicon either very closely near the eutectic composition or somewhat on the beryllium side of it in order to obtain an alloy that does not readily fracture. For this reason, I prefer to maintain the composition of the alloy at or below about 62 w/o silicon, the remainder substantially beryllium. In some cases, the addition of small amounts of other alloying elements may also be desirable, to further refine the grain size, increase the ductility, and control the microporosity.

The exceptional value of the alloy in forming mirror surfaces is demonstrated by the following example: Charges of beryllium (commercial purity) and silicon (99.9% pure) were first hot compacted to bring the materials into intimate contact and reduce oxide formation and were then melted by direct induction heating in a zirconia crucible to form an alloy having a composition of 6! w/o silicon and 39 w/o beryllium. The molten alloy was poured into a shallow cup-shaped depression approximately 6 inches in diameter formed in a graphite block to form a casting approximately seven-eighths inch thick. The casting was machined to reduce its thickness to one-half inch and its diameter to 4 inches. The disc was next ground and polished until an optical flat was obtained; it was then heated to 165 F. and its flatness tested. At this temperature, the disc exhibited a concavity of not more than one fringe shift (22-10' inches); on returning to room temperature, the disc resumed its former shape. This performance equalled or exceeded that previously obtained from the best specially prepared beryllium. A tensile test to determine the precision elastic limit indicated a permanent strain of l microinch at a stress of 3800 psi. Again, this performance was comparable to that obtained from the best specially prepared beryllium.

Although I have described the invention particularly in terms of a mirror substrate which makes fullest use of the unique properties of the alloy from which it is formed, it will be understood that the alloy is readily applied to the formation of any part which beneficially uses the numerous unique properties of the alloy material, for example, gyroscope frames and parts, among others.

From the foregoing it will be seen that I have provided an improved castable part of relatively high strength and low density that is readily castable without the formation of large porosities or shrinkage cavities which would weaken the material. The part is relatively resistant to brittle impact, is substantially isotropic in its properties, and additionally possesses a relatively high thermal conductivity in relation to its thermal expansion. The part is formed from a beryllium-silicon alloy having a microporous structure in which the constituent materials appear as finely interwoven strands of material, thereby avoiding the formation of large grains which would weaken the structure. It is also characterized by the absence of intermetallic compounds which would form brittle components which could cause it to fail under conditions of high stress.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above method without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. A cast part formed from an alloy comprising from 30% to 70% by weight of silicon, the remainder substantially beryllium characterized by a relatively high stiffness per unit of weight and an increased resistance to brittle impact as compared to either of its starting materials.

2. A cast part according to claim 2 in which said alloy is formed from approximately 50% to 62% by weight of silicon, the remainder substantially beryllium.

3. A cast part formed from an alloy of beryllium and silicon comprising from 30% to 70% by weight of silicon, the remainder substantially beryllium, characterized by a relatively high ratio of elastic modulus to density, a relatively high ratio of thermal conductivity to thermal expansion, a generally fine, microporous structure characterized by the exclusion of large shrinkage cavities in the as-cast condition, and a relatively isotropic behavior, as compared to beryllium.

4. A cast part according to claim 3 in which said alloy is formed from approximately 50% to 62% by weight of silicon, the remainder substantially beryllium.

5. A cast part according to claim 4 in which said alloy is further characterized by an increased resistance to brittle failure as compared to the primary components from which it is formed.

6. A mirror substrate formed from an alloy comprising from 30% to 70% by weight of silicon, the remainder substantially beryllium characterized by a ratio of elastic modulus to density comparable to that of beryllium.

7. A mirror substrate according to claim 6 in which the proportions of said alloy are so chosen within the stated range as to further impart to said part a relatively high ratio of thermal conductivity to thermal expansion compared to that of beryllium.

8. A mirror part according to claim 6 in which the composition of said alloy is approximately equal to the eutectic composition whereby said part is rendered substantially resistant to brittle impact as compared to the primary materials from which is is formed.

9. A mirror part according to claim 6 in which the proportions of said alloy are so chosen within the stated range as to render said part substantially isotropic throughout.

10. A mirror part according to claim 6 in which the proportions of said alloy are so chosen within the stated range as to impart to said part a microstructure comprising a relatively fine, continuous, interwoven network of fiber-like character extending throughout the structure.

1 l. A mirror part formed from an alloy comprising from approximately 30% to 70% by weight of silicon, the remainder substantially beryllium, and characterized by a ratio of elastic modulus to density and a ratio of thermal conductivity to thermal expansion comparable to that of beryllium.

12. A mirror substrate according to claim 11 in which the proportion of silicon is so chosen within the stated range as to impart to said substrate a microstructure comprising a generally continuous network of fine strands interwoven with each other in a three-dimensional interfitting spongelike array which renders said part substantially isotropic throughout and which precludes the formation of large crystals of the primary constituents of the alloy whereby the substrate is rendered readily castable without the formation of large shrinkage cavities.

13. A mirror substrate according to claim 11 in which said alloy comprises approximately from 50% to 62% by weight silicon, the remainder substantially beryllium.

14. A cast part formed from an alloy comprising from 30% to 70% by weight of silicon, the remainder substantially beryllium, characterized by a ratio of thermal conductivity to thermal expansion and resistance to brittle impact comparable to beryllium.

15. A cast part according to claim 14 in which said alloy is formed from approximately 50% to 62% by weight of silicon, the remainder substantially beryllium.

16. A manufactured part formed from an alloy comprising from 30% to by weight of silicon, the remainder substantially beryllium, characterized by an increased resistance to brittle impact as compared to its components and a relatively high stifiness per unit weight comparable to that of beryllium.

17. A manufactured part according to claim 16 in which the beryllium and the silicon are chosen within the stated range in such proportions as to impart to the alloy a generally fine, microporous structure characterized by the exclusion of large shrinkage cavities in the as-cast condition and by substantial isotropy in its mechanical properties.

18. A manufactured part according to claim 17 in which the alloy is further characterized by a ratio of thermal conductivity to thermal expansion greater than that of beryllium, the alloy being formed from approximately 50% to 62% by weight of silicon, the remainder substantially beryllium.

19. A method of forming a uniformly dense part, characterized by a resistance to brittle impact greater than that of beryllium, from a silicon-beryllium alloy, comprising the steps of:

A. preparing an alloy comprising from 30% to 70% silicon,

the balance substantially beryllium; and B. pouring the alloy, while molten, into a mold for casting to the configuration of the part desired, the casting so formed being characterized by a substantial freedom from porosity, a generally fine, microporous structure, and a ratio of elastic modulus to density and ratio of thermal conductivity to thermal expansion comparable to that of beryllium. 20. A method according to claim 19 in which said alloy comprises from 50% to 62% silicon, the balance substantially beryllium.

21. A method according to claim 19 in which the composition of said alloy is approximately the eutectic composition.

22. A method according to claim 21 in which the microstructure of the cast part is substantially as shown in FIG. 3 of the drawings. 

2. A cast part according to claim 2 in which said alloy is formed from approximately 50% to 62% by weight of silicon, the remainder substantially beryllium.
 3. A cast part formed from an alloy of beryllium and silicon comprising from 30% to 70% by weight of silicon, the remainder substantially beryllium, characterized by a relatively high ratio of elastic modulus to density, a relatively high ratio of thermal conductivity to thermal expansion, a generally fine, microporous structure characterized by the exclusion of large shrinkage cavities in the as-cast condition, and a relatively isotropic behavior, as compared to beryllium.
 4. A cast part according to claim 3 in which said alloy is formed from approximately 50% to 62% by weight of silicon, the remainder substantially beryllium.
 5. A cast part according to claim 4 in which said alloy is further characterized by an increased resistance to brittle failure as compared to the primary components from which it is formed.
 6. A mirror substrate formed from an alloy comprising from 30% to 70% by weight of silicon, the remainder substantially beryllium characterized by a ratio of elastic modulus to density comparable to that of beryllium.
 7. A mirror substrate according to claim 6 in which the proportions of said alloy are so chosen within the stated range as to further impart to said part a relatively high ratio of thermal conductivity to thermal expansion compared to that of beryllium.
 8. A mirror part according to claim 6 in which the composition of said alloy is approximately equal to the eutectic composition whereby said part is rendered substantially resistant to brittle impact as compared to the primary materials from which is is formed.
 9. A mirror part according to claim 6 in which the proportions of said alloy are so chosen within the stated range as to render said part substantially isotropic throughout.
 10. A mirror part according to claim 6 in which the proportions of said alloy are so chosen within the stated range as to impart to said part a microstructure comprising a relatively fine, continuous, interwoven network of fiber-like character extending throughout the structure.
 11. A mirror part formed from an alloy comprising from approximately 30% to 70% by weight of silicon, the remainder substantially beryllium, and characterized by a ratio of elastic modulus to density and a ratio of thermal conductivity to thermal expansion comparable to that of beryllium.
 12. A mirror substrate according to claim 11 in which the proportion of silicon is so chosen within the stated range as to impart to said substrate a microstructure comprising a generally continuous network of fine strands interwoven with each other in a three-dimensional interfitting spongelike array which renders said part substantially isotropic throughout and which precludes the formation of large crystals of the primary constituents of the alloy wHereby the substrate is rendered readily castable without the formation of large shrinkage cavities.
 13. A mirror substrate according to claim 11 in which said alloy comprises approximately from 50% to 62% by weight silicon, the remainder substantially beryllium.
 14. A cast part formed from an alloy comprising from 30% to 70% by weight of silicon, the remainder substantially beryllium, characterized by a ratio of thermal conductivity to thermal expansion and resistance to brittle impact comparable to beryllium.
 15. A cast part according to claim 14 in which said alloy is formed from approximately 50% to 62% by weight of silicon, the remainder substantially beryllium.
 16. A manufactured part formed from an alloy comprising from 30% to 70% by weight of silicon, the remainder substantially beryllium, characterized by an increased resistance to brittle impact as compared to its components and a relatively high stiffness per unit weight comparable to that of beryllium.
 17. A manufactured part according to claim 16 in which the beryllium and the silicon are chosen within the stated range in such proportions as to impart to the alloy a generally fine, microporous structure characterized by the exclusion of large shrinkage cavities in the as-cast condition and by substantial isotropy in its mechanical properties.
 18. A manufactured part according to claim 17 in which the alloy is further characterized by a ratio of thermal conductivity to thermal expansion greater than that of beryllium, the alloy being formed from approximately 50% to 62% by weight of silicon, the remainder substantially beryllium.
 19. A method of forming a uniformly dense part, characterized by a resistance to brittle impact greater than that of beryllium, from a silicon-beryllium alloy, comprising the steps of: A. preparing an alloy comprising from 30% to 70% silicon, the balance substantially beryllium; and B. pouring the alloy, while molten, into a mold for casting to the configuration of the part desired, the casting so formed being characterized by a substantial freedom from porosity, a generally fine, microporous structure, and a ratio of elastic modulus to density and ratio of thermal conductivity to thermal expansion comparable to that of beryllium.
 20. A method according to claim 19 in which said alloy comprises from 50% to 62% silicon, the balance substantially beryllium.
 21. A method according to claim 19 in which the composition of said alloy is approximately the eutectic composition.
 22. A method according to claim 21 in which the microstructure of the cast part is substantially as shown in FIG. 3 of the drawings. 